1. Field of the Invention
This invention relates to an improved image rotation system for use with a microfilm reader, and in one aspect, to an improved prism holder for mounting a dove prism in a microfilm image projector.
2. Description of the Prior Art
Microfilm readers and reader-printers have utilized a projection lens to project an image from the microfilm onto a screen or to a light-sensitive media for copying the information on the microfilm. Microfilm comes in the form of a microfiche or individual strips of film. Focussing each microfilm image requires adjustment of the projection lens along its axis, and positioning of the image in the screen some times requires rotation of the image if the image is projected at an acute angle or if some of the images are in the comic mode and other images are in the cinematographic mode. Therefore, it is common for these projections systems to utilize a dove prism which can be rotated to rotate the projected image.
U.S. Pat. No. 4,494,840, issued Jan. 22, 1985, discloses an image rotation system wherein a projection lens is used in an adjustable lens barrel to focus the image on the screen 42 and a dove prism is mounted in a prism holder in fixed position above the projection lens and in generally axial alignment therewith to afford rotation of the image by rotation of the prism holder through the use of prism gears fixed to the prism holder and a rotatable drive gear 106.
Rotation prisms are also known for use with zoom lens systems for microfilm projectors such as shown in U.S. Pat. No. 3,360,325.
Because lenses for the projection system may invariably be changed on a single reader-printer to accommodate microfilm images of various reductions, and since it is usually mandatory to focus images on a film strip or in a microfiche between one image and the other because of the manner in which they are originally photographed, the projection lens must be adjusted to obtain proper focussing. In either event, the result is the lens is moved and it is desirable that the dove prism maintain a fixed position with respect to the projection lens so that the pupil remains substantially centered on the optic axis and receives the maximum amount of light from the projection lens.
It is an object of the present invention therefore to provide a prism holder which allows the prism to float along the optic axis and adjust its position upon a change in position of the projection lens.
It is an object of the present invention to provide a prism lens holder that allows the dove prism to move axially along the axis of the prism holder and yet not to fall out of the prism holder when changing lenses.
These and other objects will become apparent upon further description of the present invention.